The invention relates to the field of recovering video and audio signals that are processed or stored separately. More particularly, the invention relates to the field of compensating for variations in time bases of audio and video signals that are processed or stored separately and synchronizing the signals for playback.
In broadcast television systems, video and audio, signals are broadcast in a composite format which includes all of the information needed to display a picture. NTSC and PAL are two widely utilized broadcast composite video standards. A composite video signal generally includes a luminance component signal and a chrominance component signal. Synchronizing pulses included in the luminance component signal synchronize the television receiver to the luminance signal. To form the composite video signal, the chrominance component signal is modulated by a high frequency subcarrier and is superimposed over the luminance component signal. A xe2x80x9ccolor burstxe2x80x9d, which is a series of eight cycles at the subcarrier frequency, appears in blanking intervals for synchronizing the television receiver to the chrominance component signal.
For broadcasting, the composite video signal is modulated by a visual carrier signal within the assigned broadcast channel and an audio signal is modulated by an aural carrier signal within the assigned broadcast channel. When a television receives the broadcast signal via a cable or antenna, the carrier signals are removed by mixing. Then, the luminance and chrominance component signals are separated from the composite signal. The luminance and chrominance component signals are then transformed into red, green and blue component signals for driving an electron gun in the television display.
When video signals are stored on a magnetic tape, such as by a video cassette recorder, the signals are not stored in a composite video format, but are stored in a format known as xe2x80x9ccolor under.xe2x80x9d In xe2x80x9ccolor underxe2x80x9d format, the chrominance and luminance component signals are processed separately. The chrominance component is heterodyned down to occupy a frequency range below the luminance component, rather than being interleaved, as in the composite video format. The luminance and chrominance component signals are then stored on the magnetic tape in tracks which are angled with respect to the length of the magnetic tape. The audio signal is frequency modulated and combined with the video tracks or is recorded on a separate longitudinal track.
Upon playback, the chrominance component is modulated back up to an appropriate frequency (3.579545 MHz in NTSC systems), reversing the xe2x80x9ccolor underxe2x80x9d process. The chrominance signal is also stabilized to correct for any time base errors caused by dimension changes in the magnetic tape or inaccuracies in the tape player mechanism. The time base errors are reduced by an analog process which utilizes a crystal controlled phase-locked loop synchronized to the color bursts. A tight time base tolerance is required by circuits in the television to be able lock onto the chrominance component signal. The luminance component signal, however, is passed directly to the television. Because the chrominance component signal has a time base that is precisely controlled and the luminance component signal does not, the component signals are no longer synchronized to each other. This results in xe2x80x9cjitterxe2x80x9d during playback which inherently degrades the picture quality.
Therefore, what is needed is a technique for compensating for time base or phase errors in video component signals for enhancing the picture quality obtainable from a video cassette player.
Further, a trend in contemporary video and computer systems is to perform a variety of digital signal processing techniques on video signals and their associated audio signals. Because each video and audio signal has its own set of characteristics, a signal processing technique utilized for one signal is not generally applicable to another signal. Therefore, the signals are often separated from each other for digital, sampling, storage and/or processing. For example, data compression techniques adapted to compress video signals are not generally suitable for compressing audio signals, whereas, data compression techniques adapted to compress audio signals are not generally suitable for compressing video signals. Further, certain processes performed on video signals for enhancing color or crispness cannot generally be applied to audio signals.
Processing of signals separately can cause problems, however, due to variations in the time base for each signal. For example, a clock signal utilized to control digital processing of a video signal stored on a magnetic tape can be locked to the sync pulses or to the color bursts in the stored video signal. Due to a varying relationship between the phase of the color bursts and the phase of the sync pulses, however, a luminance component processed according to a clock signal locked to the color bursts can suffer from +/xe2x88x921 pixel timing uncertainty. This can result in picture hopping and breakup. As another example, a chrominance component processed according to a clock signal locked to the sync pulses can suffer from jitter and poor signal-to-noise ratios.
In addition, precise timing relationships can be lost or degraded when compressing and decompressing digital audio signals. When decompressed audio signals are combined with video signals, timing variations can cause a disturbing lack of synchronism between audio and visual elements. For example, spoken words may not match movement of the speaker""s lips. Further, relatively small timing errors can cumulate over the course of a program to unacceptable levels. Accordingly, unless audio signals are properly synchronized to the associated video signals, problems can occur when recombining the signals for playback.
Therefore, what is needed is a technique for compensating for time base or phase variations in video signals and in audio signals that are separately stored or processed such that the signals can be appropriately synchronized for playback.
The invention is a method and apparatus for compensating for time base or phase errors in video and audio signals that are separately stored or processed such that the signals can be appropriately synchronized for playback. A ring oscillator locked to a crystal oscillator provides a plurality of clock signals, each having a same frequency and a slightly different phase. Each of the clock signals is applied to a multiplexor logic circuit for allowing an appropriate one of the clock signals to be selected at any given moment. By selecting appropriate ones of the clock signals in a sequence, the frequency and phase of an output clock signal appearing at the output of the multiplexor can be continuously and precisely controlled without disturbing the crystal oscillator.
Sync pulses are separated from a video signal that can have a varying time base. The sync pulses are applied to a video timing generator circuit which generates a series of digital values wherein each digital value is representative of a timing difference between an expected occurrence of a sync pulse and an actual occurrence of the sync pulse. A phase accumulator logic circuit accumulates the digital values over time for generating appropriate address signals for the multiplexer. This allows the frequency and phase of the output clock signal to be continuously and precisely controlled according to the phase of the sync pulses.
Additional logic circuits coupled to the video timing generator circuit generate a series of digital values representing a periodic signal, such as a sinusoid, having a stable time base, but which is clocked according to the clock signal appearing at the output of the multiplexor. This periodic signal can be utilized to demodulate a chrominance component signal stored according to the xe2x80x9ccolor underxe2x80x9d format such that the chrominance component signal is synchronized to an associated luminance component signal during playback by a video cassette player. The clock signal appearing at the output of the multiplexor can be utilized for separately processing video signals and associated audio signals, while maintaining time base relationships among the signals.